Adding a method to get indices of representative molecules of clusters in bitBIRCH

[TienMPhan]

Hi, can you add a method to retrieve the index of a representative molecule of a cluster in the BitBirch() class? This will help further test these molecules.

[ramirandaq]

On it! @klopezperez, let's add a simple method that retrieves the indices of the medoids of the top P most populated clusters (P given by the user).